some of the differences will depend on what type of flying you have been doing in the US and what you will be doing in the UK. If its just been around the big airfields in florida with control towers and someone talking to you on the radio all of the time, then you would probably be at home flying from Bournemouth, Cranfield, Oxford etc but might have more difficulty when it comes to places where there is no air traffic and you have to think for yourself. On the other hand if you have been used to smaller Florida fields, with just a mostly unmanned unicom, then you will fit in fine at nice friendly places like Popham, Leicester, Breighton, White Waltham etc, but might find it different going to the bigger fields.
Air law is a must, but dont get too bogged down with the pedantic stuff, or what lights balloons have to carry etc Again depending on what you are going to do a few hours with an instructor and also talking to the local pilots will hellp you out. Navigating is different, its a lot more built up overall and the roads are straight and dont all go east-west or north south, so that makes things a bit more interesting. The weather is also a lot more unpredictable and mostly worse than Florida! But at least we dont have too many hurricanes or massive thunderstorms!
Be prepared for quite a few grass runways and short runways too, maybe both not met very often in Florida.